Field observations of the effect of shear waves on sediment suspension and transport

Abstract

Field measurements of the effect of shear waves on sediment suspension and transport have been made on the seaward side of the crest of an intertidal bar using an array of pressure sensors, electromagnetic current meters and optical backscatter sensors mounted just above the bed. A tide with strong mean longshore currents (up to 0.77 m/s) was singled out for analysis, and during this tide shear waves contributed up to 93% of total velocity variance for far infragravity frequencies <0.015 Hz. Suspension events in the time series occurred at incident wave frequency, but were modulated at shear wave frequency. The offshore phase of the incident waves was preferred for suspension. The most suspension occurred when the offshore directed backwash, the undertow, and the offshore travelling part of the shear wave combined to give strong seaward directed flows >0.5 m/s. Cross-shore sediment flux due to the shear waves was directed offshore, and accounted for up to 16% of the total cross-shore transport, and up to 37% of the oscillatory cross-shore transport. The mean component dominated the cross-shore transport, accounting for up to 69% of the total. In the longshore direction, shear waves dominated the oscillatory component of transport (up to 90%). This transport was in the opposite direction to the mean flow, but was small (up to 12% of the total) compared to the mean longshore sediment transport.